Why is it that a bar of soap normally sinks but not if you put it in a soap case?

Answer 1

This is because the displacement of water to the the weight of the metal soap dish and soap is not equalized before the water reaches the top edges of the soap dish. If you had a deeper soap dish than it would most likely float. Also most likely the soap is 'stuck' to the dish so that is why when they sink it will not float on its own. Floating actually has to do with pressure rather than weight. For example a column of water 1 inch square and 1 foot tall, it weighs about 0.44 pounds depending on the temperature of the water so if you take a column of water 1 cm square by 1 meter tall, it weights about 100 grams. That means that a 1-foot-high column of water exerts 0.44 pounds per square inch [psi]. Similarly, a 1-meter-high column of water exerts 9,800Pa [pascals]. So it is the upward water pressure pushing on the bottom of the object that causes it to float. Each square inch (or square centimeter) of the object that is underwater has water pressure pushing it upward, and this combined pressure floats the object.

Answer 2

because of the wait of the soap plus the wait of the soap dish is to much for the water to carry Sort of, but here's more to the story. This is a question about specific gravity, or roughly, weight/mass per volume. Let's look at the dish. The material that the dish is made of apparently doesn't float. Or wouldn't -- If it were shaped as a solid lump it would sink. However, since it is "dished", that is slightly bowl-shaped in the top, when set upon the water, the dish displaces some of the water and "holds" a bit of air. So what you have is the dish plus that little bit of air sitting in the water. The combination dish+little bit of air weighs slightly less that the equal volume of water, so it just barely floats. This is also why a boat made of steel can float. Now add the piece of soap. The soap, which is heavier than air (which is why it isn't floating randomly around the room), displaces some of the air that's in the soap dish, the combination of dish+soap is heavier than the volume of water that it displaces, and the combination sinks.

Answer 3

Because there is air in the case which makes the total mass of the case and the soap less dense then the bar of soap is by itself.